Method for generating a gating signal for an MRI system using an ultrasonic detector

ABSTRACT

An MRI system includes an ultrasonic detector system that includes an ultrasonic transducer placed to detect movement of selected anatomic structure in a patient. The transducer signal is analyzed to produce a gating signal which is used by the MRI system to trigger data acquisition.

BACKGROUND OF THE INVENTION

The field of the Invention is nuclear magnetic resonance imaging methodsand systems. More particularly, the invention relates to the accurategeneration of cardiac gating signals for use in MR imaging andspectroscopy.

The data required to reconstruct an MR image is acquired by an MRIsystem over a period of time. In most acquisitions this time periodextends over many cardiac cycles of the patient and sometimes it isnecessary to synchronize the acquisition with the cardiac cycle. This isaccomplished in most applications by monitoring an ECG signal producedby the patient's heart and triggering, or gating, the data acquisitionsequence when the R-peak in the ORS complex is detected.

The accurate detection of the R wave peak in the ECG signal is verydifficult in an MRI system environment. First, the quality of the ECGsignal itself is seriously degraded by the magnetic induction effectscaused by the strong magnetic fields used in MRI systems. Significantinductive noise is added to the ECG signal by patient movement, heartmotion, and blood flow as well as “gradient noise” produced by therapidly changing magnetic field gradients used during MRI acquisitions.Under the best conditions the production of a reliable ECG triggersignal is very challenging.

In a large number of patients the ECG gating does not work well. Inpatients who are large with a lot of subcutaneous fat, who have chronicobstructive pulmonary disease with expanded lungs, or for other reasons,the ECG signal may be small and difficult to detect within theelectrically noisy environment of an MR imaging system. In addition, themagnetic field influences the shape of the ECG signal. This change inthe shape of the ECG signal may make it difficult to correctlysynchronize off the QRS complex. This is because the T wave may beincreased in size and become difficult to distinguish from the ORScomplex.

A solution to this problem has been to use a pulse oximeter such as thatdisclosed in U.S. Pat. No. 5,743,263 to gate from detected flow relatedchanges in the finger tip. However, the detection of systole at thefinger tip is delayed compared to systole in the ventricles. The timingbetween the two will depend on multiple factors including vascular treecompliance, vascular tree resistance, cardiac output, distance from theheart to name only a few. As a result, this often is not useful for“freezing” the motion of the heart.

In addition to cardiac imaging, there are other situations in whichcardiac gating is required. For flow imaging in peripheral vessels,frequently one wants to image these vessels when they contain themaximum amount of flow, i.e. during systole. In this situation ECG isproblematic because of the time delay problem discussed above. That is,the timing of systole at a peripheral vessel may be very different thansystole at the heart.

Another modality for producing images uses ultrasound. There are anumber of modes in which ultrasound can be used to produce images ofobjects. In the so-called “A-scan” method, an ultrasound pulse isdirected into the object by an acoustic transducer and the amplitude ofthe reflected sound is recorded over a period of time. The amplitude ofthe echo signal is proportional to the strength of scatterers in theobject and the time delay is proportional to the range of the scatterersfrom the transducer. In the so-called “B-scan” method, the transducertransmits a series of ultrasonic pulses as it is scanned across theobject along a single axis of motion. The resulting echo signals arerecorded as with the A-scan method and their amplitude is used tomodulate the brightness of pixels on a display at the time delay. Withthe B-scan method, enough data are acquired from which a 2D image of thescatterers can be reconstructed. Another way to represent ultrasoundinformation is called “M-mode”. In this technique, a single B-mode lineis reproduced repeatedly and displayed as a time plot with the ordinatecorresponding to position along the line and the abscissa representingtime. If there is motion along the line, the pixel brightnesses aremodulated as structures move in and out of or along the line of sight.Thus moving objects are well represented in this mode.

Ultrasonic transducers for medical applications are constructed from oneor more piezoelectric elements sandwiched between a pair of electrodes.Such piezoelectric elements are typically constructed of lead zirconatetitanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymercomposite. The electrodes are connected to a voltage source, and when avoltage is applied, the piezoelectric elements change in size at afrequency corresponding to that of the applied voltage. When a voltagewaveform is applied, the piezoelectric element emits an ultrasonic waveinto the media to which it is coupled at the frequencies contained inthe excitation waveform. Conversely, when an ultrasonic wave strikes thepiezoelectric element, the element produces a corresponding voltageacross its electrodes. Typically, the front of the element is coveredwith an acoustic matching layer that improves the coupling with themedia in which the ultrasonic waves propagate. In addition, a backingmaterial is coupled to the rear of the piezoelectric element to absorbultrasonic waves that emerge from the back side of the element so thatthey do not interfere. A number of such ultrasonic transducerconstructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525;4,441,503; 4,470,305 and 4,569,231.

When used for ultrasound imaging, the transducer often has a number ofpiezoelectric elements arranged in an array and driven with separatevoltages (apodizing). By controlling the time delay (or phase) andamplitude of the applied voltages, the ultrasonic waves produced by thepiezoelectric elements (transmission mode) combine to produce a netultrasonic wave that travels along a preferred beam direction and isfocused at a selected point along the beam. By controlling the timedelay and amplitude of the applied voltages, the beam with its focalpoint can be moved in a plane to scan the subject.

The same principles apply when the transducer is employed to receive thereflected sound (receiver mode). That is, the voltages produced at thetransducer elements in the array are summed together such that the netsignal is indicative of the sound reflected from a single focal point inthe subject. As with the transmission mode, this focused reception ofthe ultrasonic energy is achieved by imparting separate time delay(and/or phase shifts) and gains to the signal from each transducer arrayelement.

This form of ultrasonic imaging is referred to as “phased array sectorscanning”. Such a scan is comprised of a series of measurements in whichthe steered ultrasonic wave is transmitted, the system switches toreceive mode after a short time interval, and the reflected ultrasonicwave is received and stored. Typically, the transmission and receptionare steered in the same direction (θ) during each measurement to acquiredata from a series of points along an acoustic beam or scan line. Thereceiver is dynamically focused at a succession of ranges (R) along thescan line as the reflected ultrasonic waves are received. The timerequired to conduct the entire scan is a function of the time requiredto make each measurement and the number of measurements required tocover the entire region of interest at the desired resolution andsignal-to-noise ratio.

Cardiac gating is also used when performing ultrasonic imaging. Asdisclosed in U.S. Pat. No. 5,709,210, for example, when cardiac gatingis required a standard ECG signal is used to trigger the imageacquisition.

SUMMARY OF THE INVENTION

The present invention is a detector system for producing a gating signalfor an MRI system. More particularly, the detector system includes anultrasonic transducer positioned to insonify an anatomic structure whichmoves in response to cardiac function in a patient, a receiver connectedto the ultrasonic transducer for receiving an echo signal which isindicative of the movement of the anatomic structure and a signalanalyzer for producing a gating signal for the MRI system when apreselected characteristic in the echo signal is detected. Theultrasonic transducer may be placed, for example, on the patient's chestto detect specific cardiac movements or blood flow, or it may be locatedto detect the motion of specific peripheral blood vessels or the bloodflow in the vessels.

The invention provides a reliable gating signal for an MRI system. Theultrasonic transducer produces a signal which is less susceptible tointerference and distortion from the magnetic fields produced in an MRIsystem environment. Its amplitude is much greater than an ECG signal andit is based on physical motion of the subject rather than electricalsignals.

The invention also provides an accurate and predictable gating signal.The ultrasonic transducer may be positioned to detect movement of manydifferent structures. The structure which provides the optimal timingfor the image being acquired can thus be used as the basis for thegating signal. Such a structure may be, for example, a particular bloodvessel or a particular heart structure such as a chamber wall or heartvalve. Specific blood flow in the heart such as the atrial kick can bedetected as a marker for end-diastole. Further, respiratory gating canbe accomplished by detecting the motion of the diaphragm.

The foregoing and other objects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsherein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI system which employs the presentinvention;

FIG. 2 is a block diagram of an ultrasound detector system employed inthe MRI system of FIG. 1 to produce an ECG signal;

FIG. 3 is a block diagram of a receiver which forms part of theultrasound detector system of FIG. 2;

FIG. 4 is a pictorial representation of an alternative way to couple anacoustic transducer to a patient;

FIG. 5 is a pictorial representation of an ultrasonic image used toselect a heart wall as the detected anatomic structure; and

FIG. 6 is a pictorial representation of an ultrasonic image used toselect a peripheral vessel as the detected anatomic structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, there is shown the major components of apreferred MRI system which incorporates the present invention. Theoperation of the system is controlled from an operator console 100 whichincludes a keyboard and control panel 102 and a display 104. The console100 communicates through a link 116 with a separate computer system 107that enables an operator to control the production and display of imageson the screen 104. The computer system 107 includes a number of moduleswhich communicate with each other through a backplane. These include animage processor module 106, a CPU module 108 and a memory module 113,known in the art as a frame buffer for storing image data arrays. Thecomputer system 107 is linked to a disk storage 111 and a tape drive 112for storage of image data and programs, and it communicates with aseparate system control 122 through a high speed serial link 115.

The system control 122 includes a set of modules connected together by abackplane. These include a CPU module 119 and a pulse generator module121 which connects to the operator console 100 through a serial link125. It is through this link 125 that the system control 122 receivescommands from the operator which indicate the scan sequence that is tobe performed. The pulse generator module 121 operates the systemcomponents to carry out the desired scan sequence. It produces datawhich indicates the timing, strength and shape of the RF pulses whichare to be produced, and the timing of and length of the data acquisitionwindow. The pulse generator module 121 connects to a set of gradientamplifiers 127, to indicate the timing and shape of the gradient pulsesto be produced during the scan.

The system control 122 receives a gating signal from an ultrasonicdetector system 129. As will be described in more detail below, thedetector system 129 receives an electrical signal from an ultrasonictransducer 11 that is positioned to sense motion in the patient. Thiselectrical signal is analyzed to produce a gating signal for the pulsegenerator module 121. This gating signal serves to trigger theacquisition of NMR data using the pulse sequence prescribed by theoperator.

The pulse generator module 121 also connects to a scan room interfacecircuit 133 which receives signals from various sensors associated withthe condition of the patient and the magnet system. It is also throughthe scan room interface circuit 133 that a patient positioning system134 receives commands to move the patient to the desired position forthe scan.

The gradient waveforms produced by the pulse generator module 121 areapplied to a gradient amplifier system 127 comprised of G_(x), G_(y) andG_(z) amplifiers. Each gradient amplifier excites a correspondinggradient coil in an assembly generally designated 139 to produce themagnetic field gradients used for position encoding acquired signals.The gradient coil assembly 139 forms part of a magnet assembly 141 whichincludes a polarizing magnet 140 and a whole-body RF coil 152. Atransceiver module 150 in the system control 122 produces pulses whichare amplified by an RF amplifier 151 and coupled to the RF coil 152 by atransmit/receive switch 154. The resulting signals radiated by theexcited nuclei in the patient may be sensed by the same RF coil 152 andcoupled through the transmit/receive switch 154 to a preamplifier 153.The amplified NMR signals are demodulated, filtered, and digitized inthe receiver section of the transceiver 150. The transmit/receive switch154 is controlled by a signal from the pulse generator module 121 toelectrically connect the RF amplifier 151 to the coil 152 during thetransmit mode and to connect the preamplifier 153 during the receivemode. The transmit/receive switch 154 also enables a separate RF coil(for example, a head coil or surface coil) to be used in either thetransmit or receive mode.

The NMR signals picked up by the RF coil 152 are digitized by thetransceiver module 150 and transferred to a memory module 160 in thesystem control 122. When the scan is completed and an entire array ofdata has been acquired in the memory module 160, it is conveyed througha backplane 118 to an array processor 161 which operates to Fouriertransform the data into an array of image data. This image data isconveyed through the serial link 115 to the computer system 107 where itis stored in the disk memory 111. In response to commands received fromthe operator console 100, this image data may be archived on the tapedrive 112, or it may be further processed by the image processor 106 andconveyed to the operator console 100 and presented on the display 104.

As will become apparent from the description below, the ultrasonicdetector system 129 can take many forms. In the preferred embodiment asystem capable of producing sector scanning gray scale images is used.The ultrasonic transducer 11 is located in the bore of the magnet 141and is positioned on the patient and acoustically coupled to the anatomyof interest. The transducer is connected with a cable to the ultrasonicdetector system 129 which is located outside the scan room. Electricalshielding is employed to prevent the strong electromagnetic fieldsproduced by the MRI system from interfering with the signals produced bythe ultrasonic transducer 11.

Before the patient is placed in the bore of the magnet 141 theultrasonic transducer 11 is properly positioned to detect the movementof a chosen anatomic structure. This is accomplished by acquiringultrasonic images and moving the transducer until the chosen anatomicstructure is properly displayed in the image. The ultrasonic detectorsystem also includes a Doppler processor and the operator places therange gate for this Doppler processor over the chosen anatomicstructure. The frequency of sound waves reflecting from the identifiedanatomic structure is shifted in proportion to the velocity of thestructure; positively shifted for movement toward the transducer andnegatively shifted for movement away from the transducer. As will beexplained in more detail below, the Doppler processor computes thesefrequency shifts. These Doppler signals are used to produce the gatingsignal for the MRI, and when a reliable gating signal is obtained, thepatient is moved into the bore of the magnet 141 and the scan isperformed.

Referring particularly to FIG. 2, the ultrasonic imaging detector system129 includes a transducer array 11 comprised of a plurality ofseparately driven elements 12 which each produce a burst of ultrasonicenergy when energized by a pulsed waveform produced by a transmitter 13.The ultrasonic energy reflected back to the transducer array 11 from thesubject under study is converted to an electrical signal by eachtransducer element 12 and applied separately to a receiver 14 through aset of transmit/receive (T/R) switches 15. The transmitter 13, receiver14 and the switches 15 are operated under the control of a digitalcontroller 16 responsive to the commands input by the human operator. Acomplete scan is performed by acquiring a series of echoes in which theswitches 15 are set to their transmit position, the transmitter 13 isgated on momentarily to energize each transducer element 12, theswitches 15 are then set to their receive position, and the subsequentecho signals produced by each transducer element 12 are applied to thereceiver 14. The separate echo signals from each transducer element 12are combined in the receiver 14 to produce a single echo signal which isemployed to produce a line in an image on a display system 17.

The transmitter 13 drives the transducer array 11 such that theultrasonic energy produced is directed, or steered, in a beam. A B-scancan therefore be performed by moving this beam through a set of anglesfrom point-to-point rather than physically moving the transducer array11. To accomplish this the transmitter 13 imparts a time delay (T_(i))to the respective pulsed waveforms 20 that are applied to successivetransducer elements 12. If the time delay is zero (T_(i)=0), all thetransducer elements 12 are energized simultaneously and the resultingultrasonic beam is directed along an axis 21 normal to the transducerface and originating from the center of the transducer array 11. As thetime delay (T_(i)) is increased as illustrated in FIG. 2, the ultrasonicbeam is directed downward from the central axis 21 by an angle θ.

The time delays T_(i) have the effect of steering the beam in thedesired angle E, and causing it to be focused at a fixed range R_(T). Asector scan is performed by progressively changing the time delays T_(i)in successive excitations. The angle θ is thus changed in increments tosteer the transmitted beam in a succession of directions. When thedirection of the beam is above the central axis 21, the timing of thepulses 20 is reversed.

Referring still to FIG. 2, the echo signals produced by each burst ofultrasonic energy emanate from reflecting objects located at successivepositions (R) along the ultrasonic beam. These are sensed separately byeach segment 12 of the transducer array 11 and a sample of the magnitudeof the echo signal at a particular point in time represents the amountof reflection occurring at a specific range (R). Due to the differencesIn the propagation paths between a focal point P and each transducerelement 12, however, these echo signals will not occur simultaneouslyand their amplitudes will not be equal. The function of the receiver 14is to amplify and demodulate these separate echo signals, impart theproper time delay to each and sum them together to provide a single echosignal which accurately indicates the total ultrasonic energy reflectedfrom point P located at range R along the ultrasonic beam oriented atthe angle θ.

To simultaneously sum the electrical signals produced by the echoes fromeach transducer element 12, time delays and phase shifts are introducedinto each separate transducer element channel of the receiver 14. Thebeam time delays for reception are the same delays (T_(i)) as thetransmission delays described above, However, in order to dynamicallyfocus, the time delay and phase shift of each receiver channel iscontinuously changing during reception of the echo to provide dynamicfocusing of the received beam at the range R from which the echo signalemanates.

Under the direction of the digital controller 16, the receiver 14provides delays during the scan such that the steering of the receiver14 tracks with the direction of the beam steered by the transmitter 13and it samples the echo signals at a succession of ranges and providesthe proper delays and phase shifts to dynamically focus at points Palong the beam. Thus, each emission of an ultrasonic pulse waveformresults in the acquisition of a series of data points which representthe amount of reflected sound from a corresponding series of points Plocated along the ultrasonic beam.

The display system 17 receives the series of data points produced by thereceiver 14 through bus 22 and converts the data to a form producing thedesired image. For example, if an A-scan is desired, the magnitude ofthe series of data points is merely graphed as a function of time. If aB-scan is desired, each data point in the series is used to control thebrightness of a pixel in the image, and a scan comprised of a series ofmeasurements at successive steering angles (θ) is performed to providethe data necessary for display.

A signal analyzer 24 connects to the receiver 14 and analyzes the echosignal to detect movement of the reflectors along the beam. As will bedescribed in more detail below, the signal analyzer produces a gatingsignal for the MRI system each time a specific movement is detected.

Referring particularly to FIG. 3, the receiver 14 is comprised of threesections: a time gain control section 100, a beam forming section 201,and a mid processor 202. The time-gain control section 200 includes anamplifier 205 for each of the N=64 receiver channels and a time-gaincontrol circuit 206. The input of each amplifier 205 is connected to arespective one of the transducer elements 12 to receive and amplify theecho signal which it receives. The amount of amplification provided bythe amplifiers 205 is controlled through a control line 207 that isdriven by the time-gain control circuit 206. As is well known in theart, as the range of the echo signal increases, its amplitude isdiminished. As a result, unless the echo signal emanating from moredistant reflectors is amplified more than the echo signal from nearbyreflectors, the brightness of the image diminishes rapidly as a functionof range (R). This amplification is controlled by the operator whomanually sets eight (typically) TGC linear potentiometers 208 to valueswhich provide a relatively uniform brightness over the entire range ofthe sector scan. The time interval over which the echo signal isacquired determines the range from which it emanates, and this timeinterval is divided into eight segments by the TGC control circuit 206.The settings of the eight potentiometers are employed to set the gain ofthe amplifiers 205 during each of the eight respective time intervals sothat the echo signal is amplified in ever increasing amounts over theacquisition time interval.

The beam forming section 201 of the receiver 14 includes N=64 separatereceiver channels 210. Each receiver channel 210 receives the analogecho signal from one of the TGC amplifiers 205 at an input 211, and itproduces a stream of digitized output values on an I bus 212 and a O bus213. Each of these I and Q values represents a sample of the echo signalenvelope at a specific range (R). These samples have been delayed andphase shifted such that when they are summed at summing points 214 and215 with the I and Q samples from each of the other receiver channels210, they indicate the magnitude and phase of the echo signal reflectedfrom a point P located at range R on the steered beam (θ).

Referring still to FIG. 3, the mid processor section 202 receives thebeam samples from the summing points 214 and 215. The 1 and 0 values ofeach beam sample is a digital number which represents the in-phase andquadrature components of the magnitude of the reflected sound from apoint (R, θ). When the ultrasonic detector system 129 is in an imagingmode, a detection process indicated at 120 is implemented in which adigital magnitude M is calculated from each beam sample and output at 22to the display system, whereM={square root}{square root over (I ² +Q ²)}.

As described in U.S. Pat. No. 5,349,525 which is incorporated herein byreference, these magnitude values M are accumulated during the scan andconverted to display data that produces a two-dimensional, sector scanimage of the anatomic structures irradiated by the ultrasonic transducer11.

When switched to a Doppler mode of operation, the ultrasonic detectionsystem 129 transmits and receives echo signals from a single beam angle(θ). Referring still to FIG. 3, in this mode of operation the midprocessor 202 couples the I and Q beam samples to the signal analyzer24. The signal analyzer 24 includes a Doppler processor 222 such as thatdescribed in U.S. Pat. No. 4,217,909 issued on Aug. 19, 1980 andentitled “Directional Detection of Blood Velocities In An UltrasoundSystem”; or such as that described in U.S. Pat. No. 4,265,126 issued onMay 5, 1981 and entitled “Measurement of True Blood Velocity By anUltrasound System”. Such Doppler processors employ the phase information(φ) contained in each beam sample to determine the velocity ofreflecting objects along the direction of the beam (i.e. radialdirection from the center of the transducer 11), whereφ=tan⁻¹(I/Q).The Doppler processor 222 stores a succession of 2 msec. segments ofbeam sample data and performs a fast Fourier transformation on eachsegment. The result is a spectra signal which indicates the frequencycomponents of the echo signal.

The signal analyzer 24 processes the Doppler spectra signal to produce alogic level gating signal for the MRI system. A number of methods may beused depending on the anatomic structure selected to produce the gatingsignal. In one embodiment a high pass filter is used to remove all butthe high frequency components. The level of the frequency componentsabove the selected frequency are monitored, and if any high frequencycomponents are detected, rapid motion of the selected anatomic structureis indicated and a gating signal is produced. In another embodiment, themean value of the frequency spectrum from the Doppler processor 222 iscalculated, and a gating signal is produced when this mean value exceedsa preset frequency. In another embodiment, the gating signal may beproduced when the mean value changes rapidly, i.e. the derivativeexceeds a preset value. Such an implementation is useful in gating fromblood turbulence. The gating signal is a TTL logic level signalidentical to that produced by ECG gating systems.

When operated in the Doppler mode, the ultrasonic detector system 129periodically updates the display with a 2D sector scan Image of theirradiated anatomy. Such an image is shown in FIG. 5 where the selectedanatomy is the heart. When gating from the heart the transducer 11 ispositioned on the patient to acquire echo signals from either aperisternal short axis view or an apical long axis view. Gating signalscan be produced using heart wall motion, mitral valve motion or bloodflow. The beam angle from which a gating signal is to be acquired is setby the operator using a control panel associated with the digitalcontroller 16. The operator positions a “range-gate” on the 2D image asindicated at 226 to select the area of interest. The anatomic structureindicated by this range gate will serve as the moving structure used forgating. Such range gate positioning is described, for example, in U.S.Pat. No. 5,785,655, which is incorporated herein by reference. It is theecho signal data from the area selected by the range gate that isapplied to the Doppler processor 222 and used to produce the gatingsignal.

When used to produce a cardiac gating signal from a peripheral vessel,the ultrasonic transducer 11 is positioned on the patient to acquire a2D image of the target blood vessel as shown in FIG. 6. A range gate ispositioned in the vessel or on the vessel wall as shown at 228. Therange gate can either be positioned across the wall of the vessel todetect motion of the vessel wall, or it can be positioned within thevessel to detect pulsatile motion from blood flow. The systolic portionof the heart cycle will generate relatively high frequency shiftscompared to the rest of the heart cycle, and these can be detected bythe methods described above to provide a gating signal. This strategywill work for any vessel in which a sample volume can be positioned.

It should be apparent that the present invention is not limited tocardiac gating. In another embodiment, one may gate off the motion of amuscle, tendon, or another tissue to synchronize with imaging of amoving body part. For example, to gate the image with respiration, onecan detect motion of the liver, spleen, or diaphragm. To gate akinematic study of opening and closing the mouth, one can detect motionof the masseter muscle.

It should be apparent to those skilled in the art that many variationsare possible from the preferred embodiment without departing from thespirit of the invention. The ultrasound imaging capability describedabove is very useful as a means for “aiming” the single beam used toacquire an echo signal from selected anatomic structures for producing agating signal. However, other means may be employed to accuratelyirradiate selected anatomic structures. For example, ultrasonictransducers that produce a fixed beam may be located with fixtures orpositioned with guiding apparatus that reliably direct the beam atspecific anatomic structures. A flat transducer may be placed directlyover the carotid artery, for example, to detect wall motion or bloodflow toward and away from the transducer. Another location of interestis the point of maximum cardiac impact (PMI) on the patient's chest.

In these embodiments the transducer is placed on the patient andadjusted until a reliable gating signal is produced. No ultrasonic imageis acquired. Such an implementation would be particularly appropriatefor continuous wave (CW) Dopplers in which no image is generated. CWDopplers would most likely be restricted to blood flow detection, orsimple displacements such as motion of the patient's diaphragm.

In some situations it is desirable to remove the ultrasonic transducer11 and its associated conductors and shielding away from the bore of themagnet 141 and well outside the field of view of the MRI system.Referring to FIG. 4, in this case the transducer 11 is remotely locatedand an acoustic waveguide 26 is used to convey the ultrasonic transmitand receive beams. The waveguide 26 is made of a non-conductive materialand it extends into the bore of magnet 141. The ultrasonic transducer 11is mounted in the proximal end of the waveguide 26 and the distal end 27bears against the patient's skin and acoustically couples therewith. Alens (not shown) is formed in the distal end 27 of the waveguide 26 tofocus the ultrasonic energy at an adjustable distance therefrom.

The waveguide 26 is formed by an outer tube filled with a material thatpropagates sonic waves efficiently. Many materials can be used, but theyshould be MR inactive and preferably the tube material should propagatesound at a higher velocity than the filler in order to totally reflectsound. The waveguide 26 should also optimally prevent mode conversion inthe walls of the tube. A material such as tygon tubing may be used forthe tube and degassed, distilled water may be used for the filler.

Since the beam of sonic energy cannot be scanned or steered when usingthe waveguide 26 no standard B-mode ultrasonic image can be produced.However, an M-mode image can be generated, and it can be used toposition and orient the distal end of the waveguide 27. As those skilledin the art know, M-mode scanning was the standard method for heartultrasound imaging. As discussed above, when using certain anatomicalstructures to produce the gating signal, fixtures or guides that engageor attach to the patient may also be used to properly place and orientthe distal end 27. A more general method, however, is to use the imagingcapability of the MRI system to assist in “aiming” the waveguide 26.

Disposed around the waveguide 26 near its distal end 27 are two bands 30of MR active material. These bands 30 produce a strong NMR signal andthey appear very bright on reconstructed MR images. During setup, afluoroscopic NMR scan is conducted as described in U.S. Pat. No.4,830,012 to produce real time images of the target anatomical structureand the distal end 27 of the waveguide 26. The bright guide marksproduced in these images by the bands 30 show the location andorientation of the waveguide 26 with respect to the target structure.The operator can adjust the position of the waveguide distal end 27until the guide marks indicate that an echo signal will be acquired fromthe desired anatomic structure.

For peripheral vascular imaging, a 1 D color Doppler M-mode image may beproduced using the waveguide 26. The color Doppler image may be used toidentify vessels and used to aim the waveguide 26.

While the present invention has particular application where ECGmonitors have traditionally been used, it may also be used in many otherapplications. For example, the invention may be used to gate off theflow of blood through a selected structure. If the left ventricularoutflow tract is selected, for example, one can use the presentinvention to estimate cardiac output by measuring the mean blood flowvelocity through the tract using the ultrasonic detector and multiplyingby the cross-sectional area of the tract. The cross-sectional area ofthe tract can be measured in a scout scan using the MRI system and theultrasound detector produces the real-time mean velocity signal duringthe imaging scan. The resulting blood volume flow measurement may beanalyzed to produce one or more gating signals for the MRI system, andit may be visually indicated to attending physicians.

The present invention may also be used as a respiratory monitor toproduce gating signals indicative of patient respiration. This may bedone, for example, by locating the Doppler range gate across thediaphragm by aiming the ultrasound beam through the liver in alongitudinal orientation. When the diaphragm moves, a very highamplitude signal is produced at the output of the Doppler processorbecause the diaphragm-lung boundary produces a very large reflection.This can be used to follow respiratory rate and rhythm.

The present invention is not limited to the generation of a singlegating signal during each functional cycle (e.g. cardiac cycle). Priorart ECG monitors gate off the R-peak in the QRS complex, which occursonce per cardiac cycle. All MRI data is acquired based on this singletrigger. The ultrasound detection system of the present invention canprovide much more information about the movement, or functioning of thesubject anatomy and analysis of the echo signal may produce more thanone gating signal. For example, a first gating signal may be producedduring a first period of anatomic motion and a second gating signal maybe produced during a second period of anatomic motion. The first gatingsignal may be used by the pulse generator to acquire NMR data from thecenter k-space and the second gating signal may be used to triggeracquisition of views from the periphery of k-space.

The gating signal produced by the present invention may be more than asimple trigger signal that initiates data acquisition. For example, thegating signal may enable data acquisition for as long as It is presentand when the gating signal turns off, data acquisition is stopped. Whenapplied to cardiac imaging, for example, the gating signal is producedwhile the heart wall movement within the Doppler range gate Is within apreset limit. No assumptions are made about a data acquisition windowbased on a rhythmic heart rate, the desired acquisition window isactually detected during each heart beat.

1-33. (canceled)
 34. A magnetic resonance imaging system comprising: apulse generator to direct the magnetic resonance imaging system toperform an imaging pulse sequence to acquire magnetic resonance imagedata including data which is representative of the center of k-space anddata which is representative of the periphery of k-space; an imageprocessor to generate a magnetic resonance image of at least a portionof the patient using the magnetic resonance image data; an acoustictransducer to irradiate an anatomic structure in the patient usingultrasonic energy; a receiver to measure ultrasonic energy reflected bythe anatomic structure and to generate echo signals in response thereto;a signal analyzer, coupled to the receiver, to produce a plurality ofgating signals using the echo signals, including: a first gating signalin response to a first motion of the anatomic structure, and a secondgating signal in response to a second motion of the anatomic structure;and wherein the magnetic resonance imaging system, in response to thefirst gating signal, acquires a first set of data, and in response tothe second gating signal acquires a second set of data.
 35. The magneticresonance imaging system of claim 34 wherein one of the plurality ofgating signals initiates data acquisition of the imaging sequence by themagnetic resonance imaging system.
 36. The magnetic resonance imagingsystem of claim 34 further including: a display system, coupled to thereceiver, to convert the echo signals to ultrasonic image data andvisually display an image of the anatomic structure; and an operatorinput to permit a range gate to be positioned on the image of theanatomic structure wherein the range gate indicates an area of interestof the anatomic structure.
 37. The magnetic resonance imaging system ofclaim 34 wherein the signal analyzer includes a Doppler processor todetermine a velocity of the first motion of the anatomic structure. 38.The magnetic resonance imaging system of claim 34 wherein the signalanalyzer includes a Doppler processor to determine a velocity of thefirst and second motions of the anatomic structure.
 39. A method ofimaging a patient using magnetic resonance imaging, the methodcomprising: irradiating an anatomic structure in the patient withacoustic energy; measuring echo signals wherein the echo signals arereflections of acoustic energy from the anatomic structure; analyzingthe echo signals to detect motion of the anatomic structure; generatinga plurality of gating signals in response to detecting the motion of theanatomic structure, including: a first gating signal in response to afirst predetermined motion of the anatomic structure, and a secondgating signal in response to a second predetermined motion of theanatomic structure; collecting first image data of a magnetic resonanceimaging sequence in response to the first gating signal; and collectingsecond image data of the magnetic resonance imaging sequence in responseto the second gating signal.
 40. The method of claim 39 wherein thefirst image data is collected only while the first gating signal ispresent and the second image data is image data is collected only whilethe second gating signal is present.
 41. The method of claim 39 furtherincluding displaying an ultrasonic image of the anatomic structure usingthe echo signals.
 42. The method of claim 39 further including:converting the echo signals into ultrasonic image data; and visuallydisplaying an image of the anatomic structure using the ultrasonic imagedata.
 43. The method of claim 42 further including indicating an area ofinterest of the anatomic structure by positioning a range gate on theimage of the anatomic structure.
 44. The method of claim 39 wherein thefirst image data is image data which is representative of the center ofk-space.
 45. The method of claim 39 further including generating theplurality of gating signals in response to detecting the motion of theheart of the patient.
 46. The method of claim 39 further includinggenerating the plurality of gating signals in response to detecting themotion of the diaphragm of the patient.
 47. The method of claim 39further including generating a magnetic resonance image using the firstimage data and the second image data.
 48. The method of claim 39 whereinone of the plurality of gating signals initiates collection of imagedata of an imaging sequence and wherein the imaging sequence includescollecting the first and second image data.
 49. The method of claim 39wherein generating a plurality of gating signals in response todetecting the motion of the anatomic structure includes generating aplurality of gating signals in response to detecting the motion of thediaphragm of the patient.
 50. The method of claim 39 wherein generatinga plurality of gating signals in response to detecting the motion of theanatomic structure includes generating a plurality of gating signals inresponse to detecting the motion of the heart of the patient.
 51. Themethod of claim 39 the first image data is collected only while thefirst gating signal is present.